20 The use of wood fibers as reinforcements in composites L. M. MATUANA, Michigan State University, USA and N. M. STARK, USDA Forest Service, Forest Products Laboratory, USA DOI: 10.1533/9781782421276.5.648 Abstract:The chapter begins by discussing the major characteristics of wood in terms of structure, major chemical constituents and wood fiber production methods, which is followed by the description of major manufacturing processes for wood plastic composites (WPCs). It then reviews the effects of different variables such as the nature of wood flour (particle size, wood species, and wood flour content), filler-matrix adhesion, characteristics of polymer matrix, processing conditions, performance-enhancing additives, and foaming agents on the physical, mechanical, thermal, and durability properties of WPCs. Finally, the chapter discusses future trends in WPCs including new materials, novel manufacturing techniques, and emerging applications. Key words: wood plastic composites, wood flour, mechanical properties, durability, foaming. 20.1 Introduction: characteristics of wood Wood is classified as a lignocellulosic material. It is made up of major constituents (cellulose, hemicellulose, and lignin) and minor constituents (ash and extractives) (Baeza and Freer, 2001). The major constituents are structural components with a high molecular weight. Wood is approximately 60-75%cellulose, 20-30%lignin, 1-10%extractives and 04.5% ash (Marra, 1992).The chemical composition of wood varies between species. 20.1.1 Cellulose and hemicellulose Cellulose is the most abundant and the main structural component of wood. The cellulose molecule is largely linear. Three elements, namely carbon, hydrogen, and oxygen, are organized into ß-D-glucose units and linked together to form long linear chains that are arranged in one plane. The molecular weight of cellulose varies between 50,000 and 2.5 million depending on the origin of the sample (Fengel and Wegener, 1983). Figure 20.1 shows a combination of three cellulose building blocks. A high proportion of cellulose is crystalline, held together by intermolecular 648 © 2015 Elsevier Ltd Wood fibers as reinforcements in composites 649 20.1 A combination of three cellulose building blocks (Pettersen, 1984). hydrogen bonding. The degree of crystallinity is approximately 60-70% for pulp wood (Fengel and Wegener, 1983; Pettersen, 1984). The hydroxyl groups on cellulose are largely responsible for its reactive nature. Hydrogen bonds exist not only between cellulose hydroxyl groups, but also between cellulose and water. Cellulose is hygroscopic (attractive to water) because it is a polar molecule and easily undergoes hydrogen bonding (Marra, 1992). The absorption of water by cellulose depends on the number of free hydroxyl groups, not those linked with each other. Therefore, water cannot enter crystallites, only amorphous regions are accessible by water. The amount of swelling due to moisture absorption is limited because chain movement is limited. In addition, only hydroxyl groups present in amorphous cellulose are available for chemical reactions (Kazayawoko,1996). Hemicellulose is similar to cellulose in that it is arranged in five or six carbon sugars in chains. However, chains are relatively short, therefore are soluble or easily degraded to be soluble (Marra, 1992). The degree of polymerization maybe onlytens orhundreds of repeatingunits (Kazayawoko, 1996). Hardwoods contain more hemicelluloses than softwoods. 20.1.2 Lignin Lignin can be thought of as a matrix that holds cellulose fibers together. It is a brittle, relatively inert material that acts as both a bonding agent and a stiffening agent. Diffusion of lignin into the fiber wall increases stiffness and allows for stress transfer between matrix and fiber. Generally, softwoods have a larger percentage of lignin than hardwoods, accounting for 23-33% in softwoods and 16-25% in hardwoods, respectively (Fengel and Wegener, 1983; Pettersen, 1984). Lignin is also composed of carbon, hydrogen, and oxygen. The basic precursor is a phenolic material combined in many ways to form a highly branched, three-dimensional network (Fig. 20.2). The result is an isotropic substance. The six member rings of lignin are made up of only carbon atoms forming a stable benzene ring. In addition, there is a low occurrence of hydroxyl sites. Therefore, lignin is not as reactive as cellulose. 650 Biofiber Reinforcement in Composite Materials 20.2 Structural scheme of spruce (Picea spp.) lignin (Pettersen, 1984). 20.1.3 Extractives Wood extractives are organic substances that can be removed from wood with solvents. Extractives can include organic waxes, oils, fats, tannins, carbohydrates, acids, gums, and resins. Their greatest effects are on hygroscopicity, permeability, and durability of wood. They are deposits, meaning they are not strongly bound in the wood and are free to move (Marra, 1992). There are three types of wood extractives: aliphatic compounds, terpenes and terpenoids, and phenolic compounds. Aliphatic compounds include n-alkanes, fatty alcohols, fatty acids, fats (esters) and waxes. Terpenoids include turpentine and resin acids. Phenolic compounds include tannins, flavonoids,lignans, stilbenes, and tropolones (Kazayawoko, 1996). Extractives are typically removed through steam distillation, ether extraction, alcohol extraction, or water extraction. Steam distillation is used to remove terpenes and ether extraction is used to remove fatty alcohols, fatty acids, and waxes. Tannins are alcohol soluble while other carbohydrates and inorganic materials are water soluble (Baeza and Freer, 2001; Fengel and Wegener, 1983). Wood fibers as reinforcements in composites 651 20.1.4 Wood structure The familiar concentric ring pattern apparent on the stump of a tree is due to annual growth rings. The lighter material associated with the early growing season is called earlywood. Latewood is the darker ring and is formed later in the growing season. Earlywood is lighter in weight, softer, and weaker than latewood (Wiedenhoeft, 2010). Wood is composed of several different types of specialty cells. The majority of wood cells are elongated, pointed at the end, and oriented with the stem axis. Cell sizes are different for earlywood and latewood. Earlywood cells typically have relatively large cavities and thin walls. In contrast, latewood cells have smaller cavities and thicker walls. Cell walls are layered structures arranged concentrically and characterized by differences in chemical composition and orientation (Figure 20.3). Individual cells are glued together by the middle lamella (ML), which is 20.3 Drawing of the cell wall. Layers shown include the middle lamella(ML),theprimary wall (P), and the secondary wall in three layers (S1, S2, and S3) (adapted from Wiedenhoeft, 2010). 652 Biofiber Reinforcement in Composite Materials generally free of cellulose. The primary wall (P), secondary wall 1 (S1), secondary wall 2 (S2) and secondary wall 3 (S3), are distinguished by the orientation of cellulose fibrils in the walls. In P, cellulose fibrils are arranged in thin crossing layers, in S1 the fibrils are arranged in a slight slope perpendicular to the fiber direction, and in S2 the fibrils are roughly aligned with the fiber direction. The P and S3 layers have less cellulose present than the S1 and S2 layers. The S3 layer has a high concentration of non-structural substances. The cavity bound by the cell wall is referred to as the lumen. The change in wall thickness between earlywood and latewood is mainly due to changes in S2 thickness, which is also where the majority of cellulose is found. The density of the cell wall is approximately 1.49 g/cm3 (Marra, 1992). Softwood cells aligned in the axial direction are primarily tracheids, accounting for over 90% of the volume of wood. Hardwood cells aligned in the axial direction consist primarily of fibers, but also include vessels and parenchyma cells. Softwood tracheids can range from 1to 10 mm in length while hardwood fibers are on average 0.2 to 1.2 mm in length while (Wiedenhoeft, 2010). 20.2 Fiber processing and composite manufacturing 20.2.1 Fiber production methods Common wood filler for thermoplastics is wood flour. Wood flour is defined as a finely ground wood. It is derived from various wood planer shavings, chips, sawdust, and other clean waste wood from saw mills and residues of other wood processing industries. The production method often leads to fiber bundles rather than individual fibers.Moisture, particle size distribution, and density are monitored for consistent physical and chemical properties during processing. It is available commercially in a variety ofsize distributions and species. The most commonly used wood flour species for plastic composites in the United States are made from pine, oak, and maple. Other species are also used depending on regional availability. The main steps in its production are size reduction by several types of grinders (e.g., pulverization by crushing, Wiley mills, attrition mills, hammermill, etc.) and size classification by screening methods. The typical length-to-diameter ( L/D) ratio of wood flour ranges from about 3 : 1 to 5 : 1. Wood flour is graded by mesh size. For example, wood flour with 30 and 120 mesh corresponds to 500 and 125 µm, respectively. Detailed information on wood flour production methods is given by Clemons (2010). Wood flour is available commercially in a variety of size distributions and species. Stark and Berger (1997) reported that commercially produced